Return 404 for private GitHub repositories the caller cannot see

• Answer user and anonymous callers with 404 Not Found instead of 401 or 403 for a private repository they cannot see, on reads, writes, contents writes and admin routes, so its existence is not confirmed.
• Keep 403 for callers who can see the repository but lack the permission, and installation token errors unchanged, with regression coverage and docs.

References:

• 404 Not Found instead of 403 Forbidden for private resources requested without proper authentication, to avoid confirming that private repositories exist: https://docs.github.com/en/rest/using-the-rest-api/troubleshooting-the-rest-api
• Tokens without sufficient permissions receive 404 Not Found or 403 Forbidden: https://docs.github.com/en/rest/authentication/authenticating-to-the-rest-api

Fixes #372